There is a need within the aircraft and construction industries for long aluminium extrusions having a high and uniform fracture toughness throughout the length of the extrusion. However using conventional extrusion processes it is found that the fracture toughness may be reduced towards the back-end of the extrusion. The longer the extrusion the worse the problem becomes. In order to obtain extrusions having adequate fracture toughness along their length it is necessary either to discard a large portion from the back-end of the extrusion or for ease of production to discard a large part from the rear end of the billet. Since the size of billets is limited to those which can be accommodated by the extrusion press, the size of extrusions which can be reliably produced is also limited.
If a billet of smaller diameter than the internal diameter of the container of the extrusion press is used the extrusion ram compresses the billet along its length and the billet metal is displaced in a transverse direction to fill the container. This process is known as upsetting. It is known to increase the transverse ductility of extrusions by introducing a small degree of upset along the entire length of the extrusion billet.
U.S. Pat. No. 3455134 describes the use of tapered billets in a hydrostatic extrusion process. The billets remain tapered during extrusion and are not upset.
GB 769604 is concerned with the hot working of alloys which are not normally extrudable. A billet which may be frusto-conical is compressed axially, and then extruded with the head of the ingot extruded last. The purpose of the method is to reduce loss of metal as scrap.